In recent years, many combined cycle power plants have opted to use some form of synthesis gas (“syngas”) as a principal fuel component to increase the use of clean fuel gas derived from gasification of a cheaper solid fuel, such as coal, in a gas turbine engine or combined cycle plant. “Syngas” is the generic term given to a gas mixture that typically contains carbon monoxide and hydrogen, as well as lower molecular weight hydrocarbons such as CH4 and a substantial fraction of non-hydrocarbon components including nitrogen, carbon dioxide, H2O and oxygen. Normally, syngas also contains a significant amount of sulfur byproducts or other contaminants generated in upstream gasification operations, particularly gas compounds produced by coal gasification plants. Some, more environmentally friendly methods exist to produce syngas, such as steam reforming of either natural gas or liquid hydrocarbons. However, in all such systems, the end product has less than half the energy density of natural gas and contains hydrogen, large amounts of carbon monoxide and at least some carbon dioxide. Syngas nevertheless represents a valuable potential supplemental fuel source, particularly in combined cycle plants that include a gas turbine engine.
As noted, a major concern of most gasification systems which produce syngas, particularly those relying on coal as a primary fuel, relates to the high volume percent of carbon monoxide and carbon dioxide, as well as the presence of sulfur compounds (such as H2S and COS) and even nitrogen compounds—all of which reduce the thermal value of the syngas, create complex pollution control problems and decrease combined cycle plant efficiencies. Thus, in recent years, a number of efforts have been made, with only limited success, to reduce the amount of sulfur and other non-fuel components in the syngas feed without jeopardizing the thermodynamic efficiency of a plant or increasing the capital expenditures necessary to satisfy strict federal and state emission control standards.
One well known process for removing sulfur and other acidic gaseous pollutants in a syngas fuel stream is the “Selexol” process first developed by Universal Oil Products in the 1980s. In a Selexol system, a solvent absorbs acid gases such as H2S present in the feed at a relatively high pressure (in the range of 300 to 1500 psia) and low temperature (typically less than 40° F.). The enriched solvent containing the absorbed acid gases is then reduced in pressure and the acid gas is stripped from the solvent using steam as the heating source. In the past, the Selexol process has been successfully used to isolate and recover hydrogen sulfide and carbon dioxide as separate streams with the hydrogen sulfide being converted to elemental sulfur or used to form sulfuric acid. Despite those successes, Selexol is considered an expensive and complex alternative for eliminating sulfur and CO2 from a syngas feedstock since it involves cooling high temperature gases from the gasifier to low process temperatures.
Although some advances have been made in converting and purifying syngas produced from coal gasification, e.g., as part of an integrated gasification combined cycle (“IGCC”) plant, the commercialization of most “coal-to-hydrogen” technologies has been hindered by the high capital costs associated with removing inorganic impurities, particularly the sulfur present in domestic coal which ultimately form oxides and/or H2S that create serious environmental concerns. In addition, most known liquid absorption units for H2S involve low temperature processes that require that the entire gas stream be cooled, resulting in additional energy losses and lower efficiencies.
Apart from sulfur, the conversion of carbon monoxide and removal of carbon dioxide in combustion waste gas streams has become much more commercially significant in recent years, in part because of the economic value of converting, isolating and compressing the carbon dioxide for use in other industries or to make a “clean” carbon free exhaust release to the atmosphere. Some current CO2 capture methods rely on a fuel decarbonization process which converts carbon monoxide to carbon dioxide and removes the CO2 from the system before any combustion of fuel occurs in the power plant. However, a typical decarbonization plant is complex because it requires the use of one or more catalytic reactors and reformers as essential components. Decarbonization systems can also be thermodynamically inefficient and costly to install and operate. For example, a decarbonization process can result in an 8-12% penalty in the overall plant efficiency due to the energy required and released during the reforming process. Although CO2 can be separated from a syngas feed using a gas separation device such as permeable membranes, the separation invariably must be carried out at high temperatures and pressures in order to minimize the necessity for compressing the CO2 prior to final sequestration.
As for the hydrogen present in syngas, some conventional systems are capable of separating fuel grade H2 from a syngas feedstock but require a large number of unit operations such as multiple absorption and desorption columns and a large footprint within an existing plant. In recent years, hydrogen-selective membranes have also been used with some success to isolate the hydrogen. However, the use of membranes alone does not produce a “clean” syngas product free from residual sulfur, carbon monoxide and carbon dioxide constituents. In addition, the known hydrogen-selective membranes are not permeable to carbon monoxide and thus unable to transfer the separated gas to a fuel-rich permeate stream. (The final residual H2 and CO in the membrane retentate stream are often referred to as hydrogen “slip”). Most plant designs using hydrogen-selective membranes also require additional unit operations to ensure that the overall thermal efficiency of the plant is not degraded by the H2 and CO slip following membrane separation.
Thus, a significant need still exists in the power generation industry to create a more efficient system to effectively clean a raw syngas feed by removing unwanted sulfur byproducts, converting CO to CO2 and sequestering the CO2 without incurring the additional high energy costs and capital equipment expenditures normally required to accomplish those process objectives.